Raise when converting a margin before setup()

Replace the silent ZeroDivisionError suppression in margin.to() with
an explicit _is_setup flag. A margin must be attached to a themeable
in a theme (which calls setup()) before any unit conversion; calling
.to(...) earlier used to silently produce a margin with the new unit
label but unchanged numeric values.

Author: has2k1

plotnine/themes/elements/margin.py

@@ -4,11 +4,12 @@
 
 from __future__ import annotations
 
-from contextlib import suppress
 from copy import copy
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING
 
+from ...exceptions import PlotnineError
+
 if TYPE_CHECKING:
     from typing import Callable, Literal
 
@@ -57,6 +58,10 @@ class margin:
     Size of the figure in inches
     """
 
+    _is_setup: bool = field(
+        init=False, default=False, repr=False, compare=False
+    )
+
     def setup(self, theme: theme, themeable_name: str):
         """
         Setup the margin to be used in the layout
@@ -68,6 +73,7 @@ def setup(self, theme: theme, themeable_name: str):
         self.themeable_name = themeable_name
         self.fontsize = theme.getp((themeable_name, "size"), 11)
         self.figure_size = theme.getp("figure_size")
+        self._is_setup = True
 
     @property
     def pt(self) -> margin:
@@ -107,22 +113,23 @@ def to(self, unit: Literal["pt", "in", "lines", "fig"]) -> margin:
         """
         Return margin in request unit
         """
+        if not self._is_setup:
+            raise PlotnineError(
+                "Cannot convert a margin that has not been set up. "
+                "Call margin.setup() (or attach the margin to a "
+                "themeable in a theme) first."
+            )
+
         m = copy(self)
         if self.unit == unit:
             return m
 
         conversion = f"{self.unit}-{unit}"
         W, H = self.figure_size
-
-        with suppress(ZeroDivisionError):
-            m.t = self._convert(conversion, H, self.t)
-        with suppress(ZeroDivisionError):
-            m.r = self._convert(conversion, W, self.r)
-        with suppress(ZeroDivisionError):
-            m.b = self._convert(conversion, H, self.b)
-        with suppress(ZeroDivisionError):
-            m.l = self._convert(conversion, W, self.l)
-
+        m.t = self._convert(conversion, H, self.t)
+        m.r = self._convert(conversion, W, self.r)
+        m.b = self._convert(conversion, H, self.b)
+        m.l = self._convert(conversion, W, self.l)
         m.unit = unit
         return m
 

tests/test_margin.py

@@ -0,0 +1,60 @@
+import pytest
+
+from plotnine import element_text, theme
+from plotnine.exceptions import PlotnineError
+from plotnine.themes.elements.margin import margin
+
+
+def setup_margin(
+    m: margin, fontsize: float = 11, figure_size=(8.0, 6.0)
+) -> margin:
+    """Attach margin to a theme and call its setup() as the layout would"""
+    t = theme(
+        figure_size=figure_size,
+        plot_title=element_text(size=fontsize, margin=m),
+    )
+    m.setup(t, "plot_title")
+    return m
+
+
+def test_conversion_before_setup_raises():
+    m = margin(t=5, unit="pt")
+    with pytest.raises(PlotnineError, match="set up"):
+        m.to("fig")
+
+
+def test_conversion_same_unit_after_setup_succeeds():
+    m = setup_margin(margin(t=5, r=5, b=5, l=5, unit="pt"))
+    result = m.to("pt")
+    assert result.unit == "pt"
+    assert result.t == 5
+
+
+def test_lines_to_pt_after_setup():
+    m = setup_margin(margin(t=3, r=3, b=3, l=3, unit="lines"))
+    result = m.to("pt")
+    assert result.unit == "pt"
+    assert result.t == pytest.approx(33)
+    assert result.r == pytest.approx(33)
+    assert result.b == pytest.approx(33)
+    assert result.l == pytest.approx(33)
+
+
+def test_conversion_fig_isotropy_after_setup():
+    """Physical distances match on all sides for equal lines input"""
+    W, H = 12.0, 4.0
+    m = setup_margin(
+        margin(t=3, r=3, b=3, l=3, unit="lines"), figure_size=(W, H)
+    )
+    mf = m.to("fig")
+    assert mf.t * H == pytest.approx(mf.l * W)
+    assert mf.b * H == pytest.approx(mf.r * W)
+
+
+def test_conversion_with_zero_figure_dimension_raises():
+    """Zero figure dimension propagates ZeroDivisionError on conversion"""
+    m = setup_margin(
+        margin(t=3, r=3, b=3, l=3, unit="lines"), figure_size=(0.0, 6.0)
+    )
+    with pytest.raises(ZeroDivisionError):
+        m.to("fig")